Processing techniques developed in the semiconductor industry are now being exploited in the development of microscopic machines and sensors. Broadly known as microelectromechanical systems (MEMS), these components include microscopic motors, actuators, accelerometers, microgrippers, digital micromirror devices, and fluistors (fluidic transistor valves). Components used in radio-based communications and wireless sensing systems such as horn antennas, bolometers, high-frequency circuit probes, and other passive elements may be desirable as MEMS, for use in many applications, such as, small satellites.
Spacecraft communication systems have benefited significantly from advances made in microelectronics and very large-scale integration processes. During the past two decades the scale of integration, the materials available, the batch-production yields, the reliability, and the raw performance of high-frequency and high-speed components have steadily improved. Many frequency and speed requirements previously met by large, weighty components are now achievable by miniature, lightweight, and highly reliable devices. A wide variety of monolithic microwave integrated circuit subsystems operating between one and approximately 100 GHz have demonstrated their functional, as well as commercial, viability. With the current intense interest in exploiting the less-crowded microwave and millimeter wave frequency bands for communications, availability of small-scale structures and devices is critical. Pursuit of micro-communications subsystems will lead to reductions in weight and size, both primary components of satellite costs. MEMS technology stands to make a significant impact to obtain the smallest possible spacecraft mass while still fulfilling design objectives. Emphasis is now placed on reducing the weight of individual spacecraft subsystems. The success of MEMS and general trends toward miniaturization in such areas as propulsion, guidance, navigation, attitude control, thermal control, pressure and temperature sensing, and power could significantly benefit satellite communications systems.
It is desirable for communications satellites to use higher frequencies, to avoid not only terrestrial microwave-link congestion and noise but also traffic from other users. There are also other considerable advantages. First, the beamwidth of an antenna narrows as the frequency increases, that is, the beamwidth of an antenna is inversely proportional to both the antenna aperture and the frequency of transmission, so greater numbers of satellites can relay to the same ground antenna without interfering with each other. Second, moving to higher frequencies also allows the use of smaller onboard satellite antennas, reducing weight. At millimeter-wave frequencies, electrically large but physically small antenna structures become feasible because of the short wavelengths involved. Finally, in the 2-4 GHz C-band, limits are imposed on radiated power to prevent interference with terrestrial microwave links. These limits either do not exist or are greatly relaxed at the higher frequencies. At frequencies much above C-band, the electronics in the receiver produce most of the noise that competes with the desired signal. However, at frequencies above 10 GHz, the atmospheric absorption of RF signals causes massive propagation losses. To overcome these losses, operation at higher, less-congested frequency regimes requires not only components that deliver much higher performance, but also highly sophisticated ground stations with larger antennas. Also, oxygen and water absorption resonances occur between 60 GHz and 125 GHz, providing opportunities for intersatellite communications that are virtually immune to interference or jamming from the ground. As components of sufficiently high performance are developed and become available, it will be desirable to design satellites that take full advantage of these frequencies.
A typical communications payload is one quarter of the dry mass of a satellite. Applying micromachining technology to payloads can achieve significant savings in weight and cost. For example, a waveguide used for routing signal energy between and within subsystems, can be integrated into the bulk substrate of a microwave integrated circuit, reducing the need for external metal waveguide sections and combiners. Presently, the silicon or gallium arsenide substrate upon which microwave integrated circuits are fabricated provides a mechanical support for the active semiconductor layers and the metalization and may serve as a heat sink.
Mobile systems and dynamic communication networks can be made more compact and versatile by micromachining and exploiting unused substrate volume. Personal communications systems increasingly require the use of lightweight, low-cost receivers. A large number of compact circuits of modest performance can be produced. Micromachining technology can meet the need for integrated subsystems by using semiconductor substrate material for multilevel and buried interconnects.
The development of micromachining technology would allow inexpensive, batch-fabricated devices to be used in personal communication systems. Miniature horn and reflector antennas as well as arrays have been investigated and some have been fabricated with the use of available micromachining techniques. An integrated horn antenna for millimeter-wave applications has been suggested and a 802 GHz imaging array, double polarized antennas, monopulse antennas, and high-gain, step-profiled, diagonal-horn antennas have been proposed. The integrated horn antenna included a pyramidal horn cavity at the bottom of which is a dipole antenna. The pyramidal horn cavity is fabricated on one substrate, while the dipole antenna element is deposited on a thin membrane fabricated on a separate wafer. These two, and subsequent wafers required, are then carefully stacked, aligned and bonded or fused together to complete the antenna structure. These components offer high-frequency operation but do not include a MEMS reflector antenna having a central feed suspended entirely above the plane of the cavity aperture, all on a single wafer.
Additionally, as the frequency of operation of a subsystem increases, packaging and interconnect schemes assume critical importance. Often high performance can be achieved by advanced circuit designs which may be compromised by inefficient intrachip paths and packaging that leads to bottlenecks and losses. Communication systems presently use discrete antennas and reflectors, which are interfaced to the front-end of receiver systems via waveguide, coaxial or planar interconnects. However, these external connections to receiver circuits can inject noise into the received signal path, limiting the ability to distinguish low-level signals in the presence of noise. Reflectors and antennas typically have central feeds suspended above the reflector. While MEMS processes can release a structure to be suspended, MEMS processes have not been applied to the manufacture of integrated reflectors having central feeds suspended above the plane of the cavity aperture on a single wafer. These and other disadvantages are solved or reduced using the invention.